This invention relates to protective windows and, more particularly, to compact and lightweight protective windows. Lightweight protective windows are necessary for many reasons. A primary application is used with armored vehicles. In contrast to permanent hardened locations, such as bank teller booths, the weight of the armor plating and armored windows is often of critical importance when constructing an armored vehicle. This is particularly true when retrofitting a standard type automobile to provide a lightweight armored vehicle which does not appear to be armored. Other applications of lightweight protective windows, such as with armored self-service station modules, may advantageously use the invented protective window rather than standard safety plate glass. Often times the cost advantage derived by reducing the weight to be supported is very substantial. Furthermore, merely reducing the amount of expensive safety glass necessary to harden the target is often extremely important in reducing the overall cost.
Additionally, new types of plastics and glass have been developed and are being developed which have different uses in the halting of high velocity projectiles and bullets. Very hard and brittle safety plate glass or specially treated plastics, for example, are often placed near the outer surface of protective windows to flatten and/or fragment the incoming projectile. This type of layer is also resistant to scratching and other wear and thus is economical to maintain. More flexible and malleable layers of plastics may be placed behind such a sheet in order to stop the flattened projectile or projectile fragments without spalling (fragmenting of the protective shield). If only brittle glass or plastics are used, spalling may result in some bullets so facturing the protective window as to penetrate it. The combination of a hard outer layer with a more flexible inner layer is a more effective means of stopping incoming projectiles than the use of either layer separately.
Even without the need for lightweight protective windows, therefore, many future protective windows will utilize a plurality of layers, each layer adapted to a different purpose. Because these layers often have differential rates of expansion, the individual layers should be separated from each other to avoid damage to the protective window during temperature changes. The difficulty attendant to such vacant spaces, however, is that over long periods of time dust and moisture may become trapped therewithin. Such dust and moisture accumulations cause clouding and condensation within the vacant spaces and obscure vision through the protective windows. Because the protective windows are often permanently installed, it is difficult or impossible to remove the moisture and dust. An efficient means is needed, therefore, to guarantee that dust and moisture never enter the spaces.